Fix compatibility with Node.js v4.x
Fix compatibility with Node.js v4.x

The callback for the `message` event on the cluster is invoked
without the `worker` argument in Node.js v4.x.

This commit fixes the issue by using the `message` event that is
emitted on the workers.

palkan/websocket-shootout,palkan/websocket-shootout,palkan/websocket-shootout,palkan/websocket-shootout,palkan/websocket-shootout,palkan/websocket-shootout,palkan/websocket-shootout